An α+β type titanium alloy has been used since old days as components of aircrafts and the like by using its high specific strength. In recent years, the weight ratio of a titanium alloy to be used for aircrafts is increasing and the titanium alloy becomes important increasingly. Also in a consumer product field, an α+β type titanium alloy characterized by a high Young's modulus and light specific gravity is often used for usage for golf club faces. Further, a high-strength α+β type titanium alloy is partially used for automobile parts whose reduction in weight is regarded as important, geothermal well casings and riser pipes for offshore oil wells that require corrosion resistance and specific strength, or the like, and its further application expansion is expected.
An α+β type titanium alloy pipe product has excellent corrosion resistance and high strength, to thus be used for energy usage of the above-described geothermal well casings, pipes of oil wells, and the like. Further, a heat resistance alloy pipe product having high specific strength and having excellent high-temperature strength is used for exhaust pipes of automobiles and the like. Further, the application of the α+β type titanium alloy pipe product to strength members such as frames of high-grade automobiles and motorcycles and reinforcing members by use of its high specific strength is also promising. For this usage, strength and rigidity in the pipe longitudinal direction need to be high, and particularly, tensile strength is desirably 1050 MPa or more and a Young's modulus is desirably 130 GPa or more. Further, low manufacturing cost is more required than in the other usages.
As a method of obtaining this α+β type titanium alloy pipe, a method of manufacturing a seamless pipe using a skew rolling process is described in Patent Document 1 and Patent Document 2. In Patent Document 1, hot rolling conditions are defined and annealing is performed at a temperature equal to or higher than a β transus, and thereby fracture toughness improvement is intended. However, when annealing is performed at the β transus or higher, complete acicular structure is made and in a pipe longitudinal direction and in a circumference direction, strength and an elastic modulus become the same on a not very high level, thereby making it difficult to achieve high strength and high rigidity in the pipe longitudinal direction that is intended in the present invention.
Further, in Patent Document 2, large shear strain is introduced into the surface of a material, so that in a skew rolling process in which hot working severe for a material to be hot rolled is performed, a hot working temperature in each step is defined for the purpose of securing hot workability of the material. In this case as well, a hot-rolling texture that causes the strength in a pipe longitudinal direction high cannot be obtained, resulting in that it is difficult to achieve high strength and high rigidity in the pipe longitudinal direction that is intended in the present invention.
There is a method of obtaining a seamless pipe by a hot extrusion process using a Ugine-Sejournet process or the like other than the skew rolling process. Even by all the processes, however, it is difficult to obtain a texture capable of obtaining high strength and high rigidity in the pipe longitudinal direction. Further, as compared to a process of manufacturing a welded pipe product by bending a sheet-shaped material, productivity is low generally, so that there is also a problem of high manufacturing cost.
Next, as the method of obtaining the α+β type titanium alloy pipe, in Patent Document 3 and Patent Document 4, there is described a method in which a sheet-shaped material obtained by hot rolling or further cold rolling is subjected to bending and butt portions thereof are welded by TIG, MIG, EB, plasma arc, or the like, to thereby manufacture a welded pipe. In the both cases, as compared to the skew rolling or hot extrusion process, productivity is high, and further production yield is high because machining some portions where wall thickness is uneven, which can be often seen in a seamless pipe, is unnecessary, resulting in that it is possible to reduce the manufacturing cost.
Patent Document 3 does not require large volume cutting by defining thickness tolerance of the welded pipe to be small to thereby suppress uneven thickness in Ti-3% Al-2.5% V and Ti-6% Al-4% V (% means mass %, which will be omitted, hereinafter). Further, similarly to Patent Document 1, Patent Document 3 intends to increase the fracture toughness by obtaining a β annealed microstructure. Thus, in this case as well, the strength in the pipe longitudinal direction and the strength in the circumference direction become similar and large anisotropy in mechanical properties does not appear, so that it is difficult to achieve high strength and high rigidity in the pipe longitudinal direction that is intended in the present invention.
Further, in Patent Document 4, it is described that when a coiled sheet material called hoop is used to manufacture a titanium or titanium alloy welding pipe continuously by a roll forming method, plural welding torches are used, to thereby make it possible to achieve no defect in a weld zone and production efficiency improvement. Although in this process, the material hoop in the sheet width direction is curved to manufacture the welded pipe, the sheet width direction is not the direction in which the strength and the rigidity in the pipe longitudinal direction are increased, which will be described later.
Further, in Patent Document 5, Patent Document 6, and Patent Document 7, a heat resistant titanium alloy to be used for exhaust pipes of automobiles and motorcycles is disclosed. These alloys are each characterized by being excellent in high-temperature strength and oxidation resistance and being excellent in cold workability. However, each tensile strength at room temperature of these alloys is 400 to 600 MPa or so, and thus it is not possible to obtain 1050 MPa or more of room temperature tensile strength in the pipe longitudinal direction, which is necessary for frames of high-grade motorcycles and bicycles, strength members of automobiles, and the like.
In Non-Patent Document 1, there is described an example of the relationship between in-plane anisotropy in strength and a texture in pure titanium, and it is described that as compared to Basal-texture, (which is a texture in which a basal plane of titanium α phase, HCP, is accumulated in the normal direction of a sheet or in the direction close to the normal direction of the sheet to be referred to as B-texture, hereinafter), in-plane anisotropy in yield stress is large in Transverse-texture, (which is a texture in which a c axis orientation being the normal direction of a (0001) plane being a titanium α phase, HCP, is strongly oriented in the sheet width direction (perpendicular to the rolling and the normal direction) to be referred to as T-texture, hereinafter).
FIG. 1 each show how to show a c axis orientation being the normal direction of the (0001) plane being a basal plane of a hexagonal HCP structure in a titanium α phase. An angle between the ND axis (the normal direction of the sheet plane) and the c axis is set to θ. Further, an angle between a line obtained by projecting the c axis onto the plane of the sheet and the TD axis (the sheet width direction) is set to φ. B-texture described above can be expressed that the c axis is oriented in the direction close to the ND axis and particularly no polarization exists in the sheet plane, so that the angle θ is small and the angle φ falls in the entire circumference of −180 degrees to 180 degrees. Further, T-texture described above can be expressed that the c axis is oriented in the direction close to the TD axis, so that the angle θ is near 90 degrees and the angle φ falls near 0 degree or near 180 degrees. Further, in FIG. 1(a) and FIG. 1(b), the direction described as the RD axis (the rolling direction) is also described as the sheet longitudinal direction, hereinafter. In Non-Patent Document 1, it is described that the pure titanium is heated to a β temperature region and is unidirectionally rolled in an α temperature region, to thereby form a texture similar to T-texture. However, in Non-Patent Document 1, no explanation on an α+β type titanium alloy sheet is given. Further, in Non-Patent Document 1, no examination on effects of improvement of pipe-making properties and the like is made.
Further, in Patent Document 8, there is described a technique of starting hot rolling in a β temperature region with regard to pure titanium. This is to prevent occurrence of wrinkles and scratches by making crystal grains fine. However, in Patent Document 8 as well, no explanation on an α+β type titanium alloy sheet is given.
Further, in Patent Document 9, there is disclosed a titanium alloy containing Fe—Al. In Patent Document 10, there is disclosed a titanium alloy for golf club head containing Fe and Al, and it is described that a Young's modulus is controlled by a final heat treatment. However, in Patent Document 9, no evaluation of a texture and no examination on anisotropy in mechanical properties are made. Further, in Patent Document 10 as well, no examination is made on an effect of control of material anisotropy in a sheet plane of a hot-rolled sheet to be performed by making a texture based on hot rolling condition control.
That is, conventionally, there are no disclosures of technique related to an α+β type titanium alloy sheet capable of being worked with good pipe-making properties such that a high-strength α+β type titanium alloy pipe having an axial direction strength of 1050 MPa or more is made by forming a thin sheet material to round shape, and a high-strength α+β type titanium alloy pipe product using it.
Here, in Patent Document 11, there is disclosed a technique of increasing bendability by developing T-texture in an α+β type alloy. Further, in Patent Document 12, there is disclosed a method of manufacturing parts having high rigidity in an axial direction by machining automobile parts such as a connecting rod and an engine valve so that the axial direction corresponds to the sheet width direction by utilizing the property of the sheet where tensile strength and a Young's modulus are increased in the sheet width direction of an α+β type alloy hot-rolled plate with developed T-texture. However, there still is room to consider the conditions of hot rolling for improving the pipe-making properties, the strength and the rigidity in the pipe longitudinal direction.